SAN FRANCISCO, CA - JUNE 15: Madison Bumgarner #40 of the San Francisco Giants reacts after giving up a two-run homer to Troy Tulowitzki #2 of the Colorado Rockies in the top of the third inning at AT&T Park on June 15, 2014 in San Francisco, California. (Photo by Thearon W. Henderson/Getty Images)

Photo: Thearon W. Henderson, Getty Images

SAN FRANCISCO, CA - JUNE 15: Madison Bumgarner #40 of the San...

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DJ LeMahieu scores the go-ahead run in the eighth past the tag of the Giants' Hector Sanchez.

Photo: Jeff Chiu, Associated Press

DJ LeMahieu scores the go-ahead run in the eighth past the tag of...

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Colorado Rockies' DJ LeMahieu celebrates after scoring on Justin Morneau's two-run double off of San Francisco Giants pitcher Javier Lopez during the eighth inning of a baseball game in San Francisco, Sunday, June 15, 2014. (AP Photo/Jeff Chiu)

Photo: Jeff Chiu, Associated Press

Colorado Rockies' DJ LeMahieu celebrates after scoring on Justin...

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San Francisco Giants' Madison Bumgarner, right, is congratulated by third base coach Tim Flannery after hitting a solo home run off of Colorado Rockies pitcher Franklin Morales during the fifth inning of a baseball game in San Francisco, Sunday, June 15, 2014. (AP Photo/Jeff Chiu)

San Francisco Giants' Pablo Sandoval celebrates after hitting a solo home run off of Colorado Rockies pitcher Franklin Morales during the fourth inning of a baseball game in San Francisco, Sunday, June 15, 2014. (AP Photo/Jeff Chiu)

Photo: Jeff Chiu, Associated Press

San Francisco Giants' Pablo Sandoval celebrates after hitting a...

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SAN FRANCISCO, CA - JUNE 15: Hunter Pence #8 of the San Francisco Giants hits an RBI single, scoring Ehire Adrianza #6 in the bottom of the second inning against the Colorado Rockies at AT&T Park on June 15, 2014 in San Francisco, California. (Photo by Thearon W. Henderson/Getty Images)

Photo: Thearon W. Henderson, Getty Images

SAN FRANCISCO, CA - JUNE 15: Hunter Pence #8 of the San Francisco...

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SAN FRANCISCO, CA - JUNE 15: Madison Bumgarner #40 of the San Francisco Giants pitches in the top of the first inning against the Colorado Rockies at AT&T Park on June 15, 2014 in San Francisco, California. (Photo by Thearon W. Henderson/Getty Images)

Photo: Thearon W. Henderson, Getty Images

SAN FRANCISCO, CA - JUNE 15: Madison Bumgarner #40 of the San...

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DJ LeMahieu of the Colorado Rockies scores on a double by Josh Rutledge, putting the score at 7-4 Giants on Sunday, June 15, 2014 in San Francisco, Calif. The Rockies would go on to win the game 8-7 after scoring four runs in the eighth inning.

San Francisco Giants pitcher Javier Lopez walks off the mound during the eighth inning of a baseball game against the Colorado Rockies in San Francisco, Sunday, June 15, 2014. The Rockies won 8-7. (AP Photo/Jeff Chiu)

Photo: Jeff Chiu, Associated Press

San Francisco Giants pitcher Javier Lopez walks off the mound...

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SAN FRANCISCO, CA - JUNE 15: Home plate umpire Mike Muchlinski #76 walks away from manager Bruce Bochy #15 of the San Francisco Giants after Bochy was tossed out of the game for arguing a stike out call against one of his players in the top of the ninth inning against the Colorado Rockies at AT&T Park on June 15, 2014 in San Francisco, California. The Rockies won the game 8-7. (Photo by Thearon W. Henderson/Getty Images)

Photo: Thearon W. Henderson, Getty Images

SAN FRANCISCO, CA - JUNE 15: Home plate umpire Mike Muchlinski #76...

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Colorado Rockies pitcher LaTroy Hawkins, left, and right fielder Brandon Barnes celebrate after they defeated the San Francisco Giants 8-7 in a baseball game in San Francisco, Sunday, June 15, 2014. (AP Photo/Jeff Chiu)

Photo: Jeff Chiu, Associated Press

Colorado Rockies pitcher LaTroy Hawkins, left, and right fielder...

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Colorado Rockies starting pitcher Juan Nicasio was removed from the game early in the third inning after getting into numerous spats of trouble against the Giants on Sunday, June 15, 2014 in San Francisco, Calif. The Rockies would ultimately win the game by a score of 8-7.

Madison Bumgarner ultimately got a no decision despite giving seven strong innings against the Rockies on Sunday, June 15, 2014 in San Francisco, Calif. Reliever Juan Gutierrez would blow a 7-4 lead in the eighth inning as the Rockies went on to win 8-7.

A Giants trainer examines catcher Buster Posey after a foul-tip hit him in the face mask in the third inning against the Colorado Rockies on Sunday, June 15, 2014 in San Francisco, Calif. Posey would later leave the game.

Tyler Colvin slaps hands in the dugout after scoring in the second inning against the Rockies on Sunday, June 15, 2014 in San Francisco, Calif. The Giants went on to lose the game 8-7 after the Rockies scored five unanswered runs late in the game.

Hector Sanchez smiles in the Giants dugout after hitting a solo home run in the fourth inning against the Rockies on Sunday, June 15, 2014 in San Francisco, Calif. The Rockies would go on to win the game 8-7.

One of the Giants' best relievers is returning to a bullpen that has hit its low point of the season, off a three-game series that should have been a Giants sweep but wound up a Rockies sweep.

"It's not something you normally see in a three-game stretch like this," Javier Lopez said after the Giants squandered another late-game lead Sunday and lost 8-7 to the Rockies, capping their first losing homestand of the season (4-6).

The Giants will fly to Chicago on Monday for a two-game interleague set against the White Sox that starts Tuesday, and Casilla will be activated from the disabled list to, in manager Bruce Bochy's words, "help get things in order" after a series in which:

-- Sergio Romo entered the series with two blown saves all season and blew save opportunities both Friday and Saturday, turning 4-2 and 4-3 leads into 7-4 and 5-4 losses.

-- Juan Gutierrez hadn't surrendered a run at home in 2014, then surrendered four runs in the eighth inning Sunday.

-- Lopez had held left-handed batters to a .167 average but gave up the deciding hit in the series finale, a two-run double by lefty Justin Morneau.

"We're where we're at because of what this bullpen's done," said Bochy, whose team still owns the majors' best record despite losing six of seven. "They've been working a lot, I'll say that. So this day off is coming at a good time."

Both Romo and Jean Machi had worked three straight games, and Bochy wanted to give them the day off but used Machi in the ninth. Gutierrez got the eighth-inning call, inheriting a 7-4 lead from Madison Bumgarner, and gave up three hits and walked a batter.

He could have escaped leading 7-6, but the Giants failed to turn an inning-ending double play. That brought the Lopez-Morneau matchup, and Morneau smacked a sinker away down the left-field line.

The bullpen yielded 11 runs in nine innings, and the Giants were swept by Colorado in a three-game series for the first time since 2008. They had been 34-1 when leading after seven innings. Now they're 34-4.

"You're going to have adversity no matter who you are or how good you are," Bumgarner said. "They picked us up plenty of times. They can afford to have a bad game or two."

Bumgarner hit one of the Giants' three home runs - three of his four career homers are against the Rockies - and Hector Sanchez and Pablo Sandoval homered back to back. Sanchez pinch hit for Buster Posey, who left the game after taking a foul tip off his catcher's mask.

To clear roster space for Casilla, who had a 1.37 ERA before straining his hamstring May 21, George Kontos was optioned to Triple-A Fresno.